shrinkage - 3D print investment cast stainless

[PeterRaymond]

I'm doing a one-off stainless steel exhaust manifold to connect from my engine to a twin scroll turbocharger. Based on the space available and the advantages of an investment cast part I'm creating the manifold in Solid-Works for 3D printing a part.

In traditional investment casting the original part is cast oversize in wax to account for the shrinkage of the wax as it cools and for the shrinkage of the final part as it solidifies and cools. Using a 3D printed initial form the shrinkage of the wax should be gone, but the final shrinkage of the metal part is still there. Fortunately, the restriction in the exhaust path is mostly at the turbine, not the manifold, so I don't need perfect port matching at the inlet and outlet, but I do want to be reasonably close, which all leads to my question:

If I am 3D printing the wax,or PLA,initial part and investment casting a 300 series stainless part, what is a reasonable shrinkage to factor in?

I realize that a real number for shrinkage depends on the part geometry and is difficult to forecast, but I just need to be close.
Attached is an image of the manifold.

Thanks, Peter Raymond

 

[EdStainless]

Pull out a foundry handbook for standard shrinkage factors, 5/16" per foot.
But the problem is that you will be casting into a hot shell, so you can't just make the plug that much larger.
You have to know about the properties of the shell materials.
The stainless will shrink a lot more than the shell as they cool.
Don't make the shell too strong or you will tear the casting.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

Pattern maker's shrinkage and solidification shrinkage are different things- keep that in mind.

Pattern maker's shrinkage is typically between 1% and 4%. You can calculate a rough approximation based on the solidification temperature of your material, compared to room temperature, including characteristic lengths of your parts.

I'll also say this- while the turbine is the primary restriction, port matching can have a LARGE impact on performance because mismatches introduce turbulent and vortex flow, which is bad.

If I were going to cast my own manifold, I'd make sure the port size at the flanges was sufficient to allow for precise matching. It's not that much extra work for a large potential improvement.